Generics and Variance of Types

Generics

• Abstraction principle: avoid defining similar methods/classes with duplicate code
• Allows the specification, with a single method/class declaration, of a set of related methods/classes/types

Autoboxing and Unboxing

• Primitives types are not the same as classes and so cannot be used with generics
• Primitives are therefore auto-boxed/unboxed in Java to their corresponding wrapper class

Covariance, contravariance, invariance

Variance	Meaning
Covariant	preserves the ordering of types
Contravariant	reverses the ordering of types
Invariant	none of the above

Java Collections Framework (JCF)

• A collection is an object that represents a group of objects
• Since these objects can be of any class, we thus use a generic type to avoid defining similar methods/classes with duplicate code (recall abstraction principle)